Sensitized, stabilized evaporated silver halide films



United States Patent 3,316.096 SENSITIZED, STABILIZED EVAPORATED SILVER HALIDE FILMS Arthur A. Rasch and Wilbur C. Hodges, Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Jan. 26, 1965, Ser. No. 428,204 6 Claims. (Cl. 96-108) This application is a continuation-in-part of Rasch and Hodges US. patent application Ser. No. 361,243, filed Apr. 20, 1964, now abandoned.

This invention relates to evaporated silver halide films and more particularly to evaporated silver halide films having increased speed, and the method for their preparation.

It is known that light-sensitive photographic elements are prepared by coating a support with a film of silver halide from the vapor phase at low pressure. Such a process is described in US. Patent 1,970,496, which further suggests that prior to the vacuum deposition of the silver halide, the support may be coated with an inorganic salt such as calcium fluoride, to increase the adherence of the evaporated silver halide layer to the support. It has been found that while calcium fluoride does increase the adherence of the evaporated silver halide layer to the support, it fails to increase the speed of the evaporated silver halide film. However, insofar as applicants have been able to determine, it has never been proposed to provide light-sensitive evaporated silver halide films having a contiguous, evaporated alkali metal halide salt layer, which arrangement has been found to increase the speed of evaporated silver halide films.

One object of our invention is to provide light-sensitive evaporated silver halide films having increased speed. Another object of our invention is to provide a method of preparing light-sensitive evaporated silver halide films which have increased speed. A further object of our invention is to provide light-sensitive evaporated silver halide films which are stabilized against changes in speed. Other objects of our invention will appear herein.

1 These and other objects of our invention are accomplished by providing a light-sensitive photographic element comprising a support having coated thereon a layer of vacuum deposited silver halide and a contiguous layer of vacuum-deposited alkali metal halide salt. In accordance with our invention, evaporated silver halide films having increased speed are prepared by vacuum deposition onto a support, in any order, separate contiguous layers of silver halide and alkali metal halide salt. We have found that the photographic elements of our invention have increased speed over the previously known evaporated silver halide photographic films, and that such elements exhibit good speed stability on storage.

The method of preparing photographic elements in accordance with our invention, and the increased speed achieved with such elements, is demonstrated in Examples 1 to 4.

Example 1 Evaporated films were prepared by employing a standard vacuum coater (Consolidated Vacuum Corp. Model LC-18B), the interior metal parts of which were silver coated or made from materials that will resist corrosion by halogen gases. The materials which were evaporated were placed in boats made of thin metal strips through which an electric current was passed, heating the boats and their contents. The substrates upon which the vaporized materials were to be condensed were placed in the dome-shaped holder of the vacuum coater at some distance above the boats. In this example, three Ai-gram pellets of silver bromide were placed in a silver boat 3,316,096 Patented Apr. 25, 1967 15 x x .24 mm., and /2 gram of potassium bromide crystals was placed in a tungsten boat of the same size. The boats were clamped in the vacuum coater to separate electrodes above the base plate. Two sheets of polyester film base, 5 x 10 inches, were attached to the domed holder at a distance of about 20 inches .from the boats. A remotely operated shutter (silver coated) was placed over the boats between the substrate and the boats, and a glass monitor slide was placed between the samples of the substrate and in the same plane. The vacuum chamber was closed and evaporated to a. pressure of 5X10" torr. A potential was applied across the electrodes holding the tungsten boat causing a current of 275 amperes to flow, heating the boat'and melting the potassium bromide. When the potassium bromide was completely molten, the shutter was opened and the potassium bromide vapor was allowed to condense on the substrate for a time sufficient to form a film approximately 8 millimicrons thick. The shutter was then closed and the molten potassium bromide allowed to solidify. The silver boat was heated next by an electric current of 370 amperes, and when the silver bromide had become molten, the shutter was opened and a film of silver bromide 206 millimicrons thick was allowed to form on the substrate over the potassium bromide film. The shutter was closed, the silver bromide melt covered, and air admitted to the vacuum chamber. The film thus obtained was overcoated with a 2% solution of inert gelatin containing 0.010 gram of formaldehyde per gram of gelatin. The solution was maintained at a temperature of 45 C. and the overcoated material was air dried at room temperature. A control was made. as described above, except that the potassium bromide underlayer was omitted. Both coatings were given a sensitometric exposure, and developed for 30 seconds at 23 C. in Kodak D-72 Developer to which had been added 4 grams per liter of hypo and 0.1 gram per liter of S-mercapto-l-phenyl-1,2,3,4-tetrazole. The coatings were then fixed, washed and dried. The log exposure (in meter-candle seconds) required to produce a visible density in the control was 1.10, while only a Log E of 0.20 was required to produce a visible density on the evaporated silver halide elements of the invention. The photographic element having the potassium bromide underlayer in accordance with the invention thus had a speed increase of eight-fold over the control.

Example 2 The procedure of Example 1 was followed except that the support employed was a clay-coated paper, the potassium bromide layer was deposited at a thickness of 8 mg and the silver halide layer was vapor deposited at a thickness of 206 m This element, and a control which did not contain the potassium bromide underlayer, were sensitometrically exposed, developed and processed as described in Example 1. The Log E required to produce a visible density on the control was 0.85, while a Log E of 1.80 was needed to produce the same density on the element having the potassium bromide underlayer. Thus, the evaporated silver halide film in accordance with the invention had a ten-fold increase in speed over the control.

Example 3 The procedure of Example 2 was followed except that potassium chloride was substituted for potassium bromide, and the thickness of the silver bromide layer was 138 mu. The element thus obtained, and a control which did not contain the potassium chloride underlayer, were exposed and processed as described in Example 1. A Log E of 0.85 was required to produce a visible density on the control coating, while a Log E of 0.10 was required to produce the same density on the evaporated silver halide film having the potassium chloride underlayer. The element in accordance with the invention thus had a five to six-fold increase in speed over that obtained with the control coating.

Example 4 The procedure of Example 1 was followed except that sodium chloride was substituted for the potassium bromide, and the silver halide was vapor deposited to a thickness of 138 mp A control film was made in the same manner except that the sodium chloride layer was omitted. The elements obtained were exposed and developed as described in Example 1. A Log E of 1.15 was required to produce a visible density on the control coating, while a Log E of 0.70 was required to produce the same density on the evaporated silver halide film having the sodium chloride underlayer.

As indicated heretofore, the alkali metal salt layer may be either applied to the support and the silver halide evaporated thereover, or the silver halide may be first evaporated in the form of a thin film onto the support, and the alkali metal halide salt coating deposited by vapor deposition over the evaporated silver halide layer. Photographic elements of this type, as well as elements having the alkali metal salt deposited adjacent to the support with the silver halide layer thereover, were prepared and tested for electrical resistance, which correlates qualitatively with changes in sensitivity of evaporated silver halide films. The results are set out in Table I, which also shows the use of evaporated coatings of calcium fluoride, and the failure thereof to provide a substantial increase in resistivity. In addition, Table I shows the results obtained when a layer of alkali metal halide salt was solvent coated as an overlayer for the evaporated silver halide.

TABLE I.ELECTRICAL RESISTANCE F SILVER BRO- MIDE FILMS DEPOSITED ON VARIOUS SUBSTRATES BY VACUUM DEPOSITION Substrate Undercoat Overcoat Resistivity (ohms-om.)

N 1. 2x10 .do 5.7 l0 16 1711.4 KBL. 8. 0X10 None 1. 3 10 Casein-clay 5. 1X10 Casein-clay plus 6 mp KBr 2. 3 10 Casein-clay 1. 9X10 Casein-clay plus 16m CaF N 3. 9x10 PVA*-Ti0z 6. 8x10 PVA-TiOz plus 16 mp KBI. 1. 7X10 PVA-TiO 2. 8Xl0 PVA-TiOz plus 16 m Cal None... 9.1 10 Photo-sub do 1. 4x10 Photo-sub plus 16 my KBrdo 6. 8x10 Photo-sub 16 11111. KBT 1. 0X10 Photo-sub plus 16 m CaFz. None 1. 2x10 (Bathed in 0.1% KBr and dried) 9. 1X10 (Bathed in 1.0% KBr and dried) 1.1 10

(*Polyvinyl alcohol).

The above table also shows that it is necessary that the alkali metal salt layer be vacuum deposited rather than merely coated onto the photographic element. The increase in resistivity is negligible when an evaporated silver halide film is merely coated with a bath containing alkali metal salt. It is also shown that the calcium fluoride evaporated layers fail to produce an increase in re sistivity of evaporated silver halide films.

The unexpected nature of our invention is further illustrated in Example 5 which shows that an alkaline earth halide salt when vacuum deposited as an overcoating on a vacuum deposited silver halide film completely fails to function to increase the speed of the film.

Example 5 Following the procedure of Example 1, a layer of potassium bromide 17 me in thickness was deposited on a clay-coated paper and overcoated with a vapor deposited layer of silver bromide 137 m in thickness. Am

other element was prepared in the same manner except that calcium fluoride was substituted for the potassium bromide, and coated at a thickness of 23 m before depositing the silver bromide layer. A control coating was prepared which did not contain any layer other than the evaporated silver halide layer. All the films were overcoated with a thin protective gelatin layer, exposed and processed as described in Example 1. The control was assigned a relative speed of 100. The element hav= ing the potassium bromide undercoat had a relative speed of 625, whereas the element having the calcium fluoride undercoating had a relative speed of 80.

Example 6 illustrates the necessity for vacuum depositing the alkali metal halide salt layer.

Example 6 The procedure of Example 1 was followed, and a clay= coated paper was coated with silver bromide 137 m l in thickness, and overcoated with a layer of potassium bromide 17 m thick. A control coating which did not contain the potassium bromide overcoating was also prepared. A ortion of the same batch of potassium bro mide used to prepare the vacuum deposited layer in the photographic element of this example was added to a gelatin overcoating solution at a concentration of 10 grams per liter, and this solution was used to prepare a protective overcoating of silver bromide film 137 m thick. The photographic sensitivity of the silver bromide was not significantly changed by this treatment, and was essentially the same as the control. If the concentration of potassium bromide in the solutions used for overcoating is varied between 0.1 g./l. and 25 g./1., there is still no significant variation in the photographic sensi tivity of the silver bromide. However, the element having the vacuum deposited potassium bromide overcoating had four times the sensitivity of the control.

In an alternate experiment, a sample of the paper substrate Was bathed in a 1 percent solution of potassium bromide and dried. This material was then vapor coated with a 137 m thick film of silver bromide. The photographic sensitivity of this material was about 40% less than that coated on an untreated substrate.

Example 7 Following the procedure of Example 1, a layer of potassium bromide 11 m in thickness was deposited on a clay-coated paper and overcoated with a vapordeposited layer of silver bromide and. 206 m in thickness. An additional layer of potassium bromide 11 my. in thickness was then deposited on top of the silver bromide. A control coating was prepared which did not have any layer other than the evaporated silver bromide layer. These coatings were exposed and processed as described in Example 1. The Log E required to produce a visible density on the control was 0.90, while a Log E of was needed to produce the same density on the element having the potassium bromide layers. Thus, the evaporated silver halide film in accordance with the invention had a ten-fold increase in speed over the control.

The speed stability of light-sensitive evaporated silver halide films having a contiguous alkali metal halide layer is shown in Example 8.

Example 8 A layer of potassium bromide 8.5 m thick was deposited on St. Lawrence Clay Coated paper and an additional coating of silver bromide 137 m thick was deposited over the potassium bromide using the method and apparatus described in Example 1. A control coating was prepared consisting of a single layer of silver bromide 137 m thick. The samples received no protective overcoat but were sensitometrically exposed immediately after they were removed from the evaporator. Each coating was developed for 14 seconds at 23 C. in-

the developer described in Example 1. The sensitivity of the film with the contiguous potassium bromide layer was found to be six times greater than that of the control. Unexposed samples of these coatings were stored under room conditions and tested periodically. It was found that the sensitivity of the film of the invention remained constant for a period of at least one month. The control coating, which had no KBr underlayer, first increased somewhat in sensitivity and then decreased steadily and at one month had only one-sixth the sensitivity that it had when fresh.

The contiguous alkali metal salt layer, to effectively speed stabilize evaporated silver halide layers, must be vacuum deposited. Preferably, the halide of the alkali metal salt layer is the same as the halide of the light sensitive silver halide. For example, we prefer to use alkali metal bromide layers when the light sensitive layer is silver bromide. However, the halides of the two layers may be different, such as potassium bromide layers contiguous to a silver chloride layer. Alkali metal halide layers stabilize the speed of evaporated silver halide layers when the layers are contiguous. Thus, the alkali metal halide layer may be coated under or over, or both under and over, the light-sensitive silver halide layer. In the preferred embodiment of the invention, the alkali metal halide layer is coated under the evaporated silver halide layer, especially when the support employed is paper, such as a clay coated paper.

Any light-sensitive silver halide salt may be deposited with satisfactory results in accordance with our invention to provide satisfactory photographic films. Typical useful silver salts are silver chloride, silver bromide, and silver iodide, as well as mixed salts such as silver chlorobromide and silver bromoiodide salts. The most useful thickness of the silver halide layer is from about 100 to 1100 m Any alkali metal halide salt may be employed in accordance with the invention, the alkali metal salts of bromine and chlorine giving particularly suitable results. We prefer to use either potassium bromide or sodium chloride. In accordance with the invention, both the silver halide layer and the alkali metal salt layer are vacuum deposited by evaporating the silver halide and the alkali metal in accordance with any procedure known in the art. If desired, the photographic element may contain an evaporated silver halide layer with a contiguous overcoating and undercoating of alkali metal halide. The most useful thickness of the evaporated alkali metal halide salt layer is from about 5 to 75 m The supports which may be employed to prepare the photographic elements of the invention include all the conventional photographic supports with particularly good results being achieved with glass and supports such as paper, preferably paper coated with, for example, a casein-clay mixture or a pigmented (e.g., titanium dioxide) polyvinyl alcohol coating.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

We claim:

1. A photographic element comprising a support having coated thereon separate contiguous layers of vacuum deposited light-sensitive silver halide and vacuum deposited alkali metal halide salt.

2. The method of preparing a photographic element which comprises vacuum depositing on a support, in any order, a layer of silver halide and a separate, contiguous layer of alkali metal halide salt.

3. Claim 1 wherein the alkali metal salt is selected from the group consisting of potassium bromide and sodium chloride.

4. A photographic element comprising a support having coated thereon separate contiguous layers of vacuum deposited silver halide at a thickness of from 100 to 1100 mi, and a vacuum deposited layer of alkali metal halide salt at a thickness of 5 to mu.

5. A photographic element comprising a support having vacuum deposited thereon a potassium bromide layer 5 to 75 m thick, and a light-sensitive silver halide layer vacuum deposited over the potassium bromide layer, said silver halide layer being from about to about 1100 III/1. thick.

6. The photographic element of claim 5 wherein the silver halide is silver bromide.

OTHER REFERENCES Clark, P. V. McD., et al.: Experiments on Photographic Sensitivity, Jour. Phot. Sci., vol. 4, 1956, pp. 3, 12-13.

NOR MAN G. TORCHIN, Primary Examiner. C. E. DAVIS, Assistant Examiner. 

1. A PHOTOGRAPHIC ELEMENT COMPRISING A SUPPORT HAVING COATED THEREON SEPARATE CONTIGUOUS LAYERS OF VACUUM DEPOSITED LIGHT-SENSITIVE SILVER HALIDE AND VACUUM DEPOSITED AKALI METAL HALIDE SALT. 